Wool Tuft test

On the autospeed.com site there’s an article that was one of the reasons for developing my rear window spoiler. It’s an article about aerodynamic testing on a Volkswagen New Beetle. They are using short pieces of wool (tufts) taped to the car at certain strategic points. See this article: Autospeed Aero Testing – Part 3.

I’ve always wanted to do the same to my beetle. So when I finally had some time ( the last day of my holiday.. ) I gathered my family and got to work…

I cut a bunch of wool tufts and taped them to the rear of the car (with the help of my daughter ).

The plan was to first test it with the spoiler on. Then remove the spoiler and do the test again. The test track was a piece of highway
between my hometown and a city nearby, about 10 kilometers long.

10 Comments

Good job!
This is something I know I’ve wanted to try and never made the effort 🙁

“The more I learn about (basic) aerodynamics the less I know”… So much to learn. So little time 🙂

No question -IMHO- that the airflow had detached at the point of your spoiler, as expected. The yarn looks like it’s doing a great job of visualizing (for us) the air flow. It’s really slick to see a low-tech-low-cost method of understanding the hidden air flow.

In the 1971 my family bought a Super Beetle wit AC. my mother & I picked it up at the Porsche plant in Stuttgart. I was 14 & we drove it around Europe for a while before bringing it home to Charleston, South Carolina (USA). One of the AERO tricks that we tried was a thin, sheet metal, matt black spoiler which mounted on thin, sheet metal stands on the body vent below the rear window. This made the car more stable when passing semi-trucks at about 130 KPH. The highway gas millage also increased 3-4%. This, being at the top of the rear window, seems like it would do even better.

Very cool, that must have been quite an adventure driving through Germany in 1971. Germany has a lot of very beautiful areas.
Once a year we do a weekend holiday with our beetle in Germany (we live close to the German border). We really like Germany as a holiday destination.

Something that I wonder is being factored in here. remember that a New Beetle is a rebodied Vw Golf; a front engine watercooled car. The “old” Beetle is of course air cooled and relies heavily on air drawn in through the vents below the back window.
Surely, therefore, we want to direct air into this area rather than away from it (airflow separation). The suction from the vent area is going to reduce the boundary layer significantly anyhow I should think? In terms of rear airflow, the engine and it’s cooling fan are a major factor: Air goes in through the rear vent, through the engine, and is expelled at pressure through the rear beneath the apron and guards, reducing rear drag. This is an effect that wind tunnel tests of the Beetle rarely take into account because they are static tests with the engine switched off!
It would be interesting to play with spoiler shapes above and below the cooling vent to see what effect it has on overall airflow dynamics.

I think you over-estimate the amount of air the engine is sucking in. Compared to the fast amount of air passing over the car at, say, 80 km/h, it’s probably only a small percentage.
If it was comparable, you wouldn’t be able to stand behind a beetle without experiencing a storm-like wind coming from under the car.

Um
if your trying to increase mpg the Tufts pointing up mean there’s more drag therefor decreasing mpg.
You want the air as close to car as long as possible on the rearend. That’s why the best aerodynamic shape is a teardrop type shape.

I disagree. The rear of the beetle is not like the teardrop shape, it’s too steep.
The ideal angle is at about 15 degrees, probably a bit like the rear end of a Toyota Prius. The VW beetle rear is much steeper.
If you keep the air attached to the car as long as possible to the rear end on a beetle, it increases drag.

Hey there,
I really liek what you’re onto. I flirted with the subjet myself a few years back. My main unanswered question was and still is “Does a spoiler on the back window – or a bar type above it (as remelle) – has an incidence on the amount of air entering the engine compartment, therefore on the engine cooling?”
Martin Harris raised the problem above. I can give you an approach of what I got into with my own road testing back then.
On a 1971 1302, a little lower than origin, stiff suspension, shorter white springs, shortened struts and bislstein all the way. Car had 5 speed gearbox and a 1745cc with a 36DRLA. Car was in the 5.5-6.5 L/100km with 185 on front and 205 in the back.
With the front vents slightly opened, Windows roll down by 1 to 2 cm, with chromed rain shields above window fresh air in the car, increased consumption, no heat change on the engine. Freeway test, between 90-130 km/h.
Vents half to fully opened, same speeds, got 20 to 30 °C up.
Solve that issue with a front (unfanned) oil cooler Under a vented appron. Reduced the increase of temp to 10-20 degrees.
Closed vents and Windows, opened rear popouts, 10 to 20°C increase in temp. Done after ram air front cooler installed. This preventing me from putting a front spoiler BTW.
No change in temp with a metal sun roof such as yours on the 1303.
Never got to do the tests for the rear spoilers. But I do think there is a reason why there are no above wings on 911 for a reason.
Some 993 have a 3rd brake light in a kinda spoiler just above the deck lid though. But I think it’s mainly for California 3rd break ligth régulations.

You did some thorough testing!
So, what you are saying is a disturbance in airflow from the side of the beetle had a greater effect then a disturbance on the roof of the beetle? Interesting..
Might be something to do with the way the air curves inwards from the sides at the back of the car after it passes the sides.

Two things to remember though:
1. There is no airflow “into” the engine compartment in the stock configuration. The fan’s job is to suck air into the engine compartment.
There is a low pressure area behind the beetle that starts just above the rear vents. See this image:http://images.gerrelt.nl/roofspoiler/beetle_windtunnel_test.jpg
2. Don’t compare the 911 with the beetle too much in this case. The rear of the 911 has a rear sloping back that more closely resembles the raindrop shape, which is the ideal aerodynamically shape.
That’s probably the reason why you don’t see the above wings on a 911.
The rear of the beetle is too steep, that’s why it can benefit from a roofspoiler.
See also this page:http://www.gerrelt.nl/section-aerodynamics/aerodynamics-roofspoiler-explanation